Although gasifiers are a long standing technology, they have yet to seriously challenge our use of fossil fuels due to their inherent complicated nature with respect to the processing of feedstock. Because gasification is an inexact science, engineers have been unsuccessful to date in creating a process or product that can be turned over to an operator to be utilized as a predictable machine. Variations in feedstock, even within homogeneous species of feedstocks, yield varying results.
The existing art consists of several styles of gasifiers. Most are of the style that was originally constructed many decades ago in which a bed of feedstock is heated within a vessel, or stages within multiple vessels, where feedstock is subjected to heating at either atmospheric or raised pressure, and is in the presence of steam, oxygen, air, and/or some other gas to provide for a gasification reaction of the feedstock. The gasifier can be directly heated by combustion, which is most common, or indirectly heated by another source.
The original belt type or moving bed gasifier is still in use in some applications, where a dry feedstock is heated while a conveyor belt moves the feedstock through a heating zone. The feedstock breaks down over time, and ash is left over at the end of the machine. Synthetic Natural Gas (syngas) is released and captured within the reactor.
Up flow, down flow, and cross flow gasifiers consist of a large vessel in which the feedstock is carried through a reaction zone by gravity or by motive steam, air, or another fluidizing injection agent. These are typically more efficient, but again require dry feedstock.
Another major type of gasifier is the “tumbler” style gasifier, where a rotating drum rolls feedstock within it to expose the feedstock to a heating medium that is either entrained within, or is applied to the exterior of the tumbling tube. These gasifiers are not very applicable in large scale operations, and require the feedstock to dry out which leads to pyrolysis and gasification.
Yet another method is an entrained flow gasifier, where the feedstock is injected into a vessel, usually with steam or air, but which has also been entrained with hydrogen or inert gases. The flow is entrained with the fluidizing agent, and these are typically heated indirectly. The existing art includes entrained flow gasification where the feedstock is dried prior to entrainment.
Some styles include a devolatilization reactor preceding the gasifier which dries and extracts the light gaseous materials from the feedstock. This stabilizes the reaction by consuming the entrained oxygen prior to gasification to eliminate “run away” reactions where the feedstock material combines with the entrained oxygen and pyrolyzes and creates poor quality syngas.
All of these methods have a single major shortfall, which is that the controllability of their processes is subject to a fixed mechanical dimension, and the ability to change the operating characteristics of these gasifiers is limited by this.
Other methods of handling waste include composting, digestion, incineration, and supercritical water processes. Composting is expensive, has a pollution potential based on runoff, and has long breakdown times for the waste. Incineration is expensive, has high pollution potential and often faces public opposition. Supercritical water processes require equipment suitable for processes that run at over 3,000 psi. The combination of very high pressure and exothermic reactions can be dangerous to operate. In addition, supercritical processes produce hydrogen fuel only as carbon is consumed as CO2.
Accordingly, there is a need in the art for improved gasification systems and related methods that overcome these issues.